1986 Volume 40 Issue 3 Pages 295-303
When coated papers are provided to multi color unit type offset printing, uneven transfer between the ink film of cyan and that of magenta occurs sometimes resulting the ink mottle on printed surface.
In this paper, the correlation between the local variation of basis weight in coated paper and that of magenta ink density on the printed surface and also, the relation between latex distribution and trapping were studied. Latex distribution was determined by using EPMA technique presented in the previous report.
The results were summarized as follows:
(1) In order to measure the local variation of basis weight in the specimen, contact β-ray method was employed. The variation of the basis weight correspond to that of the optical density in the X-ray film.
It was recognized that the pattern of basis weight variation corresponded to that of magenta ink density. From the results, it was assumed that the trapping trouble was dependent upon the coating weight variation which had been caused by the local variation of basis weight in base paper under various coating conditions.
(2) In order to correspond to the size and pattern of ink mottle on the printed surface, continuous measurement of Os X-ray intensity with EDS was carried out by counting the unit area of 1×1 mm2 from 30 to 50 spots on 0s04 tagged coated layer surface. Color distribution maps of Os were depicted indicating the levels of OsLα X-ray intensities in 12 colors (2600-4700 counts).
It was shown by the color distribution maps of Os that the specimen which caused good trapping had considerably uniform distribution of latex but that with poor trapping had ununiform distribution of latex corresponding to the size and shape of ink mottle.
(3) The similar results were also obtaind by the color distribution map of Os measured with CMA (Computer aided X-ray microanalyzer).
(4) The Os distribution map of cross section measured with CMA indicated that the specimen with good trapping had less migration of latex into base sheet than that with poor trapping.
(5) From the results, it was recognized that the pattern of basis weight variation corresponded to that of latex distribution on the surface and ink mottle originated from poor trapping during unit type offset printing of coated paper was caused by the ununiform distribution of latex occuring at low binder level on the surface.
